Provide a more useful error for invalid use case.

The previous help statement was not very usefull for users who
don't know how to use nbconvert (ironically, the exact set of
users who would encounter the error).

IPython/nbconvert/nbconvertapp.py

@@ -1,230 +1,228 b''
 #!/usr/bin/env python
 """NBConvert is a utility for conversion of .ipynb files.
 
 Command-line interface for the NbConvert conversion utility.
 """
 #-----------------------------------------------------------------------------
 #Copyright (c) 2013, the IPython Development Team.
 #
 #Distributed under the terms of the Modified BSD License.
 #
 #The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 #Imports
 #-----------------------------------------------------------------------------
 
 # Stdlib imports
 from __future__ import print_function
 import sys
 import os
 import glob
 
 # From IPython
 from IPython.core.application import BaseIPythonApplication, base_aliases, base_flags
 from IPython.config import catch_config_error, Configurable
 from IPython.utils.traitlets import (
     Unicode, List, Instance, DottedObjectName, Type, CaselessStrEnum,
 )
 from IPython.utils.importstring import import_item
 
 from .exporters.export import export_by_name, get_export_names, ExporterNameError
 from IPython.nbconvert import exporters, transformers, writers
 from .utils.base import NbConvertBase
 
 #-----------------------------------------------------------------------------
 #Classes and functions
 #-----------------------------------------------------------------------------
 
 nbconvert_aliases = {}
 nbconvert_aliases.update(base_aliases)
 nbconvert_aliases.update({
     'format' : 'NbConvertApp.export_format',
     'notebooks' : 'NbConvertApp.notebooks',
     'writer' : 'NbConvertApp.writer_class',
 })
 
 nbconvert_flags = {}
 nbconvert_flags.update(base_flags)
 nbconvert_flags.update({
     'stdout' : (
         {'NbConvertApp' : {'writer_class' : "StdoutWriter"}},
         "Write notebook output to stdout instead of files."
         )
 })
 
 
 class NbConvertApp(BaseIPythonApplication):
     """Application used to convert to and from notebook file type (*.ipynb)"""
 
     name = 'ipython-nbconvert'
     aliases = nbconvert_aliases
     flags = nbconvert_flags
     
     def _classes_default(self):
         classes = [NbConvertBase]
         for pkg in (exporters, transformers, writers):
             for name in dir(pkg):
                 cls = getattr(pkg, name)
                 if isinstance(cls, type) and issubclass(cls, Configurable):
                     classes.append(cls)
         return classes
 
     description = Unicode(
         u"""This application is used to convert notebook files (*.ipynb)
         to various other formats.""")
 
     examples = Unicode(u"""
         The simplest way to use nbconvert is
         
         > ipython nbconvert mynotebook.ipynb
         
         which will convert mynotebook.ipynb to the default format (probably HTML).
         
         You can specify the export format with `--format`.
         Options include {0}
         
         > ipython nbconvert --format latex mynotebook.ipnynb
         
         You can also pipe the output to stdout, rather than a file
         
         > ipython nbconvert mynotebook.ipynb --stdout
         
         Multiple notebooks can be given at the command line in a couple of 
         different ways:
   
         > ipython nbconvert notebook*.ipynb
         > ipython nbconvert notebook1.ipynb notebook2.ipynb
         
         or you can specify the notebooks list in a config file, containing::
         
             c.NbConvertApp.notebooks = ["my_notebook.ipynb"]
         
         > ipython nbconvert --config mycfg.py
         """.format(get_export_names()))
     # Writer specific variables
     writer = Instance('IPython.nbconvert.writers.base.WriterBase',  
                       help="""Instance of the writer class used to write the 
                       results of the conversion.""")
     writer_class = DottedObjectName('FilesWriter', config=True, 
                                     help="""Writer class used to write the 
                                     results of the conversion""")
     writer_aliases = {'FilesWriter': 'IPython.nbconvert.writers.files.FilesWriter',
                       'DebugWriter': 'IPython.nbconvert.writers.debug.DebugWriter',
                       'StdoutWriter': 'IPython.nbconvert.writers.stdout.StdoutWriter'}
     writer_factory = Type()
 
     def _writer_class_changed(self, name, old, new):
         if new in self.writer_aliases:
             new = self.writer_aliases[new]
         self.writer_factory = import_item(new)
 
 
     # Other configurable variables
     export_format = CaselessStrEnum(get_export_names(),
         default_value="full_html",
         config=True,
         help="""The export format to be used."""
     )
 
     notebooks = List([], config=True, help="""List of notebooks to convert.
                      Wildcards are supported.
                      Filenames passed positionally will be added to the list.
                      """)
 
     @catch_config_error
     def initialize(self, argv=None):
         super(NbConvertApp, self).initialize(argv)
         self.init_notebooks()
         self.init_writer()
 
     def init_notebooks(self):
         """Construct the list of notebooks.
         If notebooks are passed on the command-line,
         they override notebooks specified in config files.
         Glob each notebook to replace notebook patterns with filenames.
         """
 
         # Specifying notebooks on the command-line overrides (rather than adds)
         # the notebook list
         if self.extra_args:
             patterns = self.extra_args
         else:
             patterns = self.notebooks
 
         # Use glob to replace all the notebook patterns with filenames.
         filenames = []
         for pattern in patterns:
             for filename in glob.glob(pattern):
                 if not filename in filenames:
                     filenames.append(filename)
         self.notebooks = filenames
 
     def init_writer(self):
         """
         Initialize the writer (which is stateless)
         """
         self._writer_class_changed(None, self.writer_class, self.writer_class)
         self.writer = self.writer_factory(parent=self)
 
     def start(self):
         """
         Ran after initialization completed
         """
         super(NbConvertApp, self).start()
         self.convert_notebooks()
 
     def convert_notebooks(self):
         """
         Convert the notebooks in the self.notebook traitlet
         """
         # Export each notebook
         conversion_success = 0
         for notebook_filename in self.notebooks:
 
             # Get a unique key for the notebook and set it in the resources object.
             basename = os.path.basename(notebook_filename)
             notebook_name = basename[:basename.rfind('.')]
             resources = {}
             resources['unique_key'] = notebook_name
             resources['output_files_dir'] = '%s_files' % notebook_name
 
             # Try to export
             try:
                 output, resources = export_by_name(self.export_format,
                                               notebook_filename, 
                                               resources=resources,
                                               config=self.config)
             except ExporterNameError as e:
                 print("Error: '%s' exporter not found." % self.export_format,
                       file=sys.stderr)
                 print("Known exporters are:",
                       "\n\t" + "\n\t".join(get_export_names()),
                       file=sys.stderr)
                 sys.exit(-1)
             # except Exception as e:
                 # print("Error: could not export '%s'" % notebook_filename, file=sys.stderr)
                 # print(e, file=sys.stderr)
             else:
                 self.writer.write(output, resources, notebook_name=notebook_name)
                 conversion_success += 1
 
         # If nothing was converted successfully, help the user.
         if conversion_success == 0:
 
             # No notebooks were specified, show help.
             if len(self.notebooks) == 0:
-                self.print_help()
+                self.print_examples()
 
-            # Notebooks were specified, but not converted successfully.  Show how
-            # to access help.
-            else:
-                print('For help, use "ipython nbconvert --help"')
+            # Show how to access help.
+            print('For help, use "ipython nbconvert --help"')
 
 
 #-----------------------------------------------------------------------------
 # Main entry point
 #-----------------------------------------------------------------------------
 
 launch_new_instance = NbConvertApp.launch_instance